With the advent of high speed xerographic copy reproduction machines wherein copies can be produced at a rate in excess of three thousand copies per hour, the need for a document handler to feed documents to the copy platen of the machine in a rapid, dependable matter was recognized to enable full utilization of the reproduction machines potential copy output. A number of document handlers are currently available to fill that need. These document handlers must operate flawlessly to virtually eliminate the risk of damaging the originals and generate minimum machine shutdowns due to uncorrectable misfeeds or documents multifeeds. It is in the initial separation of the individual documents from the document stack where the greatest number of problems occur.
Since the documents must be handled gently but positively to assure separation without damage through a number of cycles, a number of separators have been suggested such as friction rolls or belts used for fairly positive document feeding in conjuction with a retard belt, pad, or roll to prevent multifeeds. Vacuum separators such as sniffer tubes, rocker type vacuum rolls, or vacuum feed belts have also been utilized.
While the friction roll-retard systems are very positive, the action of the retard member, if it acts upon the printed face can cause smearing or partial erasure of the printed material on the document. With single sided documents, this does not present a problem as the separator can be designed so that the retard mechanism acts upon the underside of the document. However, with documents printed on both sides, there is no way to avoid the problem. Additionally, the reliable operation of friction retard feeders is highly dependent on the relative frictional properties of the paper being handled. This cannot be controlled in a document feeder.
In document hanlers where the document set may be circulated a plurality of times, the document handler is normally provided with a bottom sheet separator-feeder to allow feeding of documents while documents which have already been copied to be returned to the top of the document stack. In this way after all the documents have been copied, they are in the correct order for recirculation if necessary.
One of the major problems with bottom sheet feeders is that without knowing how large a stack of documents is to be placed in the feed tray or the paper weight of the individual documents, it is difficult to design a sheet separator that is gentle enough for small stacks of light weight paper and still be capable of handling large stacks of heavy weight paper.
It is therefore the object of this invention to provide a document handler that automatically compensates for variable weight sheets without complicated controls or mechanisms.